Touch screen displays are becoming ubiquitous in electronic devices such as tablets, electronic book readers (e-book readers), and many others. In these electronic devices, the display screen itself becomes both an output device and an input device. End users of such devices expect high performance of both input and output functionalities. As an output device, users expect to be able to view the display from any orientation and over a broad spectrum of ambient lighting conditions (e.g., across a spectrum from indoor environments with low ambient lighting to outdoor environments in direct sunlight). As an input device, users expect a robust, reactive touch screen that works in the first instance of touching the screen. As a result, consumers will not settle for a compromise in performance of either input or output functionality of a touch screen; they want optimal performance in both. For example, if the touch sensor of the display degrades optical quality to any noticeable degree, the device becomes suboptimal from a consumer perspective. At the very least, consumers will not tolerate functional failure of the device, which may be caused by breakdown of the materials used for individual components of touch screen. For instance, exposure to ultraviolet (UV) radiation may accelerate failure conditions through UV degradation (e.g., increased brittleness) of the materials used for various components of the electronic device, such as the touch sensor substrate.
Some touch screen devices utilize indium tin oxide (ITO) to provide a conductive, yet transparent, layer for sensing touch-based input. ITO-based touch sensors may be suitable in small form factor devices (e.g., devices with 5 inch diagonal displays and smaller), but ITO has limitations in larger form factor devices. For example, in tablets and other “large” touch screen devices (e.g., devices larger than 10 inch diagonal displays), there is a tradeoff between good conductance and good transparency when a device is constrained to using ITO-based touch sensors because thicker ITO is needed for suitable conductance, which results in poor optical quality of the touch sensor.
In addition, conventional touch sensors typically utilize materials such as polyethylene terephthalate (PET) for a touch-sensor substrate. PET is known to exhibit unsuitably high birefringence that degrades the optical quality of the touch sensor, which, in turn, lowers the optical performance of the display. In some instances, PET and like materials can cause colorations across the field of view of the display that resemble a rainbow-like artifact on the displayed image. This can be exacerbated when a user is wearing polarized sunglasses, which may result in a rainbow-like artifact across the entire display surface. PET can also exhibit relatively high haze, low transmittance, and/or discernable color that negatively impacts the optical performance of the display. Moreover, permanent discoloration (e.g., yellowing) of the PET is known to occur under UV radiation (e.g., sunlight).